Process for producing ordered complex perovskite-type crystals with high dielectric constants



Sept. 2, 1969 s. F. GALASSO 3,464,785

PROCESS FOR PRODUCING ORDERED COMPLEX PEROVSKITE-TYPE CRYSTALS WITH HIGHDIELECTRIC CONSTANTS Filed Dec. 14, 1965 2 Sheets-Sheet l l IIIHH IHllllll I I IIHH] I I I lllll] l IIHHII 106 EKJ/JT/V/T/ f p l nnuq IIHHH] IIIIHH] Z20 fi & 600 00 /006 /Zfl0 TEMPf/PA 7.065 G l l I Sept, 2,1969 Filed Dec. 14, 1965 s. F. GALASSO 3,464,785

PROCESS FOR PRODUCING ORDERED COMPLEX PEROVSKITE-TYPE CRYSTALS WITH HIGHDIELECTRIC CONSTANTS 2 Sheets-Sheet 2 United States Patent O W PROCESSFOR PRODUCING ORDERED COMPLEX PEROVSKITE-TYPE CRYSTALS WITH HIGH DI-ELECTRIC CONSTANTS Salvatore F. Galasso, Manchester, Conn., assignor toUnited Aircraft Corporation, East Hartford, Conn., a

corporation of Delaware Filed Dec. 14, 1965, Ser. No. 513,735 Int. Cl.C01g 35/00; Bld 9/00; H01]: 3/02 US. CI. 23-51 3 Claims ABSTRACT OF THEDISCLOSURE Perovskite-type single crystals of the formula 'o.3a o.s7) swherein A is barium or strontium and B is magnesium, calcium, zinc,nickel or cobalt are prepared by mixing powders of a compound yieldingthe oxide of A with Ta O and the B metal oxide and then mixing thepowders with a flux of the formula AF wherein A is as defined above. Theresulting mixture is heated to a temperature above the melting point ofthe flux and after holding for at least 0.5 hours is gradually cooled toeffect crystal formation.

This invention relates in general to materials characterized by high,temperature-stable dielectric constants. It contemplates the productionof ordered complex perovskite-type crystals with dielectric constantswhich are relatively unaffected thermally up to temperatures as high as800 C.

In the electronics industry the more common capacitor compounds whichare dielectnically temperaturestable, exhibit dielectric constants onthe order of approximately 10. Those materials, including theferroelectrics such as barium titanate, which possess dielectricconstants several orders of magnitude larger than the usual capacitorcompounds, are known to be temperature sensitive. A need exists in theindustry for capacitor insulators displaying both high dielectricconstants and reasonable thermal stability, particularly in the highertemperature ranges.

Accordingly, it is a fundamental object of this invention to providematerials exhibiting high electrical resistivity properties over a widetemperature range.

A further object of this invention is to provide ordered complexperovskite-type crystals characterized by high dielectric constantswhich are substantially independent of their environment up totemperatures of 800 C.

An additional object is to provide methods of producing crystals of highelectrical resistivity from compounds identified by the formula A(B B )Oparticularly those characterized by the formula A(B Ta )O These andother objects and advantages of this invention will be set forth in thefollowing description or will be evident therefrom or from practice ofthe invention.

FIGURE 1 is a graph of log resistivity versus temperature for arepresentative Ba(Mg Ta )O single crystal.

FIGURE 2 is a graph of dielectric constant versus temperature for aBa(Mg Ta )O single crystal.

The perovskite structure is recognized in the art as that adopted bymany ABOg-type compounds in which large A ions and oxygen ions formclose-packed layers with small B ions in the octahedral holes betweenthe oxygen ions. Investigations have been made of a large number of newcompounds of the general formula A(B' B" )O where B and B are twodifferent elements in the octahedrally coordinated cation position ofthe perovskite structure. Experiments have confirmed that many com-Patented Sept. 2, 1969 pounds indentified by the formula A(-B' B )O formin an ordered perovskite structure. Inorganic Chemistry, 2, 482 (1963).Complete ordering has been found in those materials of the A(B' Ta )-Otype, which can :best be described as a hexagonal unit cell containingthree close-packed BaO layers with the B and Ta ions arranged in theoctahedral holes.

Compounds were prepared with the divalent B ions selected to obtaincompounds with differences ranging from 0.01 to 0.52 in the radii of theB and Ta ions, and where A was a barium or strontium ion. The structuraldata for the A(B Ta )O compounds prepared, including cell sizes, ionicradii of the divalent B ions, and the difference in ionic radii of the Bions is presented in Table I.

TABLE I [Structure data for A(B' Ta 0 compounds] Diff. in ionic Ionicradii of radii of B Compound Cell size, A. B ion, Ax ions, A.

go.as au.e1) s 0. 67 0. 0 0.s3 0.01) 0a 69 01 (G u.aa -o.e1)Pa 73 050.aaT8a.c-1) 03 74 06 o.asTao.e1)Oa 12 (Cdu.aaTao.o1)0a (1:4.167 97 29 Mmaa M'DOa 99 31 b0.aa o.o1)Oa a=4.250 1. 20 52 goas o.u1) 03 0. 67 01 S(N u.aaTao.o1)Oa 69 01 '(C 0.3a 0.67)O3 .-{Z:;$ .73 .05 s 0.asTau.l7)O3{g:g;gg .74 .06 S (Cao.aaTao.o1)0s 99 31 Single crystals of the generaltype A(B Ta )O' were prepared and were found to possess uniqueelectrical properties. As representative of these unique properties,reference is made to FIGURES 1 and 2 in the drawings. Single crystals ofthe compound,

for example, where A is a barium ion and B is a magnesium ion, exhibitan electrical resistivity greater than 10 ohm/cm. at C., maintaining aresistivity of more than 10 ohm/cm. up to 800 C. Further, as best seenin FIGURE 2, this crystal is characterized by a dielectric constant ofapproximately 500 which is relatively independent of temperature overthe above-mentioned temperature range. This was completely unexpectedinasmuch as similar electrical resistivity measurements in powdercompacts of substantially the same composition did not displayequivalent electrical properties. While the exact reasons for thesuperior electrical properties of the crystal are not known, acomparison of the respective X-ray patterns associated with the variouscrystals showed that the perovskite pseudo-cells were less distorted inthe crystal. It is postulated that the presence of a small amount offluoride ion in the crystals produced, the fluoride ion being present inthe flux in which the crystals were grown, is a contributing factor.

The preferred technique for growing crystals of the A(B Ta )O' type isas follows, the production of the Ba(Mg Ta )O crystals being specifiedfor the sake of simplicity:

Reagent grade BaCO Ta O and MgO were dry-mixed in the approximateproportions to obtain stoichiometric compositions with the amount offluoride flux as indicated in- Table II; The fluoride flux, BaF wasselected in the preparation of the-Ba(B' Ta )O3 type crystals, insteadof the more commonly used KF, PbO- or P130- PbF fluxes, in order tominimize the amount of cation impurities introduced into the'crystals..Wliile BaCOgwas used in the preparation of thecrystalsdiscussed, other compounds; such as Ba'NO from which bariumoxide may be derived, are use'able in the process. Similarly, althoughMgO was used in the preparation of the Ba(Mg Ta )0 crystals,- in otherinstances the diva? lent metal oxide, BO, was used; corresponding to theB ion which: was to be included in the compound prepared. Thecarbonatesg,nitrates ancl other sources of B'O are also useable in thepreparation of these compounds.

The mixtures were placed in 50ml. covered platinum crucibles and heatedto a temperature approximately 100 C. above the meltingpointof. theflux. Soaking at temperature was'efiec'tedfor periods ranging from 0.5to 8.5 hours, depending on the composition of the mixture. Uponcompletion of soaking, the-crucibles and contained samples weregradually cooled over a temperature range of approximately 400 CI. and.the resulting crystals-Were mechanically extracted from the flux. Thespecific soaking temperatures, soaking times, sample Weights, flux ratesand cooling rates are given in' Table IT. While the tabled conditionsrepresent the best of numerous experiments designed to increase the sizeof the crystals, it will be. understood that these conditions arerepresentative only. Appropriate variations in these parameters are wellwithin Crystals prepared in accordance with the abovedescribed'techniqueswere'nearly cubic in shape and varied in color from yellow to"green. Selected crystals from each batch were analyzed using X-raydiffraction tech-- niques and photographs: were taken using a 57.3 min;radius Philips X-ray' camera and" high intensity copper Ka radiation,and the presence of ordering and cell sizes were determined.

For the purpose of investigating. the electrical resistance andcapacitance propertiesv of the crystals, gold electrodes were placed ontwo faces of the largest crystals and the specimens were-mounted-betweenopposing platinum electrodes in a resistance-heated furnace. The DC.resistance of each crystal was then measured as a function oftemperature by. monitoring the current while a constant voltage wasapplied across the faces. Capacitance measure ments were obtained with aBoonton Model 74C-58 capacitance bridge using a frequency of 100 kc. Inthe electrical measurements made, no hysteresis eifect was uncovered".

The high resistivities andlarge dielectric constants of these crystals,which were seen to be essentially invariant with temperature, make themattractive as dielectrics in electronic microcircuitry which may besubjected to temperature variations andtemperature extremes.

While the present invention has been d'escribedin connection withseveral preferred embodiments and conditions, no limitation is intendedthereby except as defined in the following claims.

What is claimed is:

1. The method of preparing ordered complex perovskite-type crystals ofthe general formula holding the mixture at the soaking temperature forat least 0.5 hour;

gradually cooling thev mixture to effect crystal formation;

and extracting the crystals from the flux.

2. The method of preparing ordered complex perovskite-type crystalsidentified by the formula having a high tempe'rature stabl'e dielectricconstant comprising:

mixing powders consisting of I B'aCO ,Ta O and MgO in the proportions toobtain a stoichiometric composition according to the formula Ba(Mg Ta)O' mixing the powders with a BaF flux;

heating the mixture in a covered platinum container to a soakingtemperature" above the melting" point of the flux;

holding the mixture at the" soaking temperature for gradually coolingthe mixture to effect c'rystal'forn'iation;

and extracting the crystals from the flux.

3'. The method of claim 2 in which the mixture is heated to a soakingtemperature approximately C. above the melting point' of the flux, andheld at the soaking temperature for approximately 8.5 hours, and themixture is gradually cooled over a temperature range'of approximately400 C. to effect crystal formation.

References Cited UNITED STATES PATENTS HERBERT T. CARTER, PrimaryExaminer US. Cl. XJR. 25263.5

